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The fridge shouldn't kill the battery. The positive feed for the fridge should be connected directly (well, via a fuse) to a separate pin on the 12S connector from all of the other caravan electrics. If the fridge operates off 12v with the caravan detached from the car, then the caravan wiring is wrong (not that all fridges have the courtesy of an indicator light.) If the fridge runs (from the caravan battery) when hooked up to the car but with the engine turned off, then the split charge wiring has not been correctly implemented in the car.
There have been a few changes to towbar wiring over the years, and as Netman says, most pertain to the 12S. The significant changes that I'm aware of are:
Later caravans require only a permanent live feed and an ignition live feed for the fridge. Earlier ones have a separate ignition live feed to charge the battery, but the later ones use the permanent live, switched by a relay from the fridge feed. This connection is redundant on the later wiring, but does no harm if it's present.
Later caravans require a separate earth for the fridge, and this is done (if I recall correctly) via the centre pin on the 12S. If this is not earthed in the car then the fridge won't work in the 'van when towing. The connection is redundant on a caravan with old wiring, but once again, it doesn't matter if it's there.
There's another difference that pertains to the 12N connector, but which only applies on rather old towcars. Before the 12S connector was introduced, there was a permanent live feed to the caravan on the 12N for running the 12V systems when hooked up to the car on site. This feed was moved to the 12S and the pin on the 12N reassigned for the fog light feed. The upshot is that if you plug a trailer with a rear fog light into a car with the original spec of wiring, the fog light will stay on all the time (and drain the battery if you leave it hooked up for any length of time while parked.)
So it should be possible to have a 'universal' 12S socket that will work with both old and new caravans. It should provide the ignition switched battery charging feed for the older caravan wiring, and a separate earth for the fridge for the newer caravan wiring.
Just to make matters more interesting, there are a couple of new standards too. The most common is a single socket with thirteen pins which combines the functions of both the 12N and 12S. There's another, more unusual design which has the conventional pins of the 12N with the supplimentary connections around the collar. This means that you can connect a normal 12N plug to the socket and still get all of the road lighting. I do wonder what happens to the supplementary connections if you use a metal 12N plug though.
Currently in Horatio's garage:
2010 Laguna III Initiale.